An image including a finger print and/or a palm print (hereinafter referred to as a “finger/palm print”) is subjected to predetermined image processing to recognize the finger/palm print image, in a known finger/palm print image processing technique.
JP-A-2002-99912 discloses an example of the finger/palm print image processing technique. In the image processing method according to this technique, noise is removed from an input image to restore an original image. The technique includes: a process for dividing an input image into plural blocks; a process for performing a Fourier transformation on each of the blocks obtained by the block dividing means; a process for estimating a power of an original image and a power of noise at each frequency, for setting a gain by use of estimation values of the two kinds of powers so as to take a smaller value as a relative size of the power of the noise at the frequencies increases greater, and for suppressing the power of the noise by multiplying an amplitude at a frequency corresponding to Fourier component information by the gain; a process for performing inverse Fourier transformation on a Fourier component the amplitude of which has been changed; and for combining images of the respective blocks transformed by the inverse Fourier transformation means, to construct an entire image after restoration.
This publication describes that according to this technique, a noise component is reduced by using a function which neglects very small components for image data after a Fourier transformation. Thereafter, an inverse Fourier transformation is performed to restore an original image, and thus, a ridgeline can be emphasized. The publication also describes that directions of respective pixels and regions are not examined. Hence, even if the size of very small blocks as units for dividing an image is set to be relatively large supposing a wide interval between stripe patterns, there is no adverse influence that measurement of a directivity in regions having a narrow interval between stripe patterns fails.
Among conventional finger/palm print image processing techniques, examples of techniques for judging a region of a finger/palm print image are described in JP-A-2003-44856, JP-A-8-110949, and JP-A-8-129644. In a background separation processing in a finger/palm print image comparison device described in JP-A-2003-44856, an input finger/palm print image is blurred by a Gauss transformation and then binarized, and a region transformed into black pixels is judged to be a region including a finger/palm print while a region transformed into white pixels is judged to be a background region.
In an image quality judgment method for a finger/palm print image according to a finger/palm print input method described in JP-A-8-110949, an input image is binarized and is judged as having a good image quality if the input image is binarized into black pixels at a ratio between 0.4 to 0.55. In another image quality judgment method for a finger/palm print image described in JP-A-8-129644, an input image is binarized and expressed by thin lines. End points and branch points are extracted. If density of the end points and branch points falls within a predetermined range, the input image is judged as having a good image quality.
Among conventional finger/palm print image processing techniques, examples of techniques for extracting ridgelines of a finger/palm print are described in JP-A-8-110945, JP-A-9-167230, JP-A-2002-288641, and JP-A-2002-288672. In an image quality judgment method for a finger/palm print image described in JP-A-8-110945, a finger/palm print image is binarized by obtaining a binary threshold value from a histogram of pixel values for each small region, thereby to extract a ridgeline image. In finger/palm print image processing devices described in JP-A-9-167230, JP-A-2002-288641, and JP-A-2002-288672, an input image is subjected to a frequency analysis for each small region, to detect plural ridgeline candidates. For each region, a substitute expressing a correct ridgeline is selected from the ridgeline candidates, based on continuities to ridgelines in peripheral regions, to extract a ridgeline image.
The technique described above for judging a region of a finger/palm print image involves a possibility of deterioration in accuracy of a region judgment if the background includes any contrasting density such as a dirt or noise. The other technique also described above for extracting ridgelines of a finger/palm print involves a possibility that correct ridgelines cannot be extracted and a fine structure of finger/palm print ridgelines cannot be extracted if a finger/palm print includes a wrinkle or scratch.
Patent Document 1: JP-A-2002-99912
Patent Document 2: JP-A-2003-44856
Patent Document 3: JP-A-8-110949
Patent Document 4: JP-A-8-129644
Patent Document 5: JP-A-8-110945
Patent Document 6: JP-A-9-167230
Patent Document 7: JP-A-2002-288641
Patent Document 8: JP-A-2002-288672
Non-Patent Document 1: Ohtsu Noriyuki, “Automatic Threshold Select Method Based on Discriminant and Least Square Criteria”, The Institute of Electronics, Information and Communication Engineers Transactions, Vol. J63-D, No. 4, pp. 349-356